Continuous data protection (CDP), also called continuous backup, is a storage system in which all the data in an enterprise is backed up whenever any change is made. It's an important advance in backup technology, but whether it is a complete substitute for other forms of backup software depends on what you mean by "backup."

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CDP systems provide running backups that are constantly updated as the data is changed. Approaches to CDP range from specialized mirroring to block-by-block copies, with running logging of the data.

CDP copies are highly granular. You can restore data to within minutes or to the exact point in time needed. CDP systems also keep multiple copies of the data so if the system is corrupted, it can be rolled back to just before the corruption happened. Often the affected files can be restored without touching the others.

The robustness and flexibility of CDP has led some proponents to claim that CDP systems can completely replace other forms of backup. Their reasoning goes like this: Since you can roll back to any point in time, what's the point of making conventional backups?

The problem is that modern backups fulfill three distinct functions. A pure CDP system backup does extremely well at one of these tasks, does another fairly well and does the third poorly.

Task #1 is the immediate recovery of damaged or lost data. This is the most common use for backup and it is the job that CDP does best.

Task #2 is true backup -- keeping copies of information for a limited period of time (for example, up to 90 days) to recover in case of a problem. CDP can accomplish this task fairly well, although it may require a lot of disk space.

Task #3 is archival storage, where copies of data are kept for years or even indefinitely. Because of its reliance on disk storage, CDP is a poor choice for this task.

Furthermore, an emerging role for backup, one which is closely related to archival storage, is retention for compliance. Here the data is kept for regulatory or legal reasons for a period mandated by either law or good sense. In some cases, the data must be kept in a read-only form to prevent tampering -- something that is nearly impossible to achieve with CDP.

Most CDP vendors recognize the multiple roles of backup and don't try to sell pure CDP as a complete replacement for other forms of backup. Instead, many CDP vendors design their products to work with tape and other long-term backup approaches. For example, when Network Appliances Inc. acquired Alacritus Software last year, it inherited the company's Chronospan software product, which provides the benefits of both CDP and tape.

Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last 20 years he has been a freelance writer specializing in issues related to storage and storage management.

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